In the production of recycled paper from used paper with print thereon, it is necessary to effect deinking. Deinking is often accomplished utilizing a froth flotation process, typically by using a large vat (e.g. 20-60 feet in diameter) into which a suspension having a consistency of about 0.8-1.5% is introduced. A typical vat for this purpose is shown in U.S. Pat. No. 4,331,534 (the disclosure of which is incorporated by reference herein). While the froth flotation practiced utilizing this technology is practical, it does not have flotation rates as high as desired, and does not lend itself to a totally enclosed pressurized system. With increasing interest in the production of recycled paper, it is desirable to increase the throughput and efficiency of the deinking (flotation) procedure, which can be a limiting stage of the recycled paper production process.
According to the present invention, deinking of recycled paper cellulosic pulp at enhanced throughput and efficiency levels is accomplished by utilizing hydrocyclones in the froth flotation process. Hydrocyclones, such as shown in U.S. Pat. No. 4,279,743 and 4,838,434, have been known for use in froth flotation in the mining industry for at least a decade, however the possible applicability thereof to deinking during the manufacture of recycled paper was not contemplated, no doubt due in part to the differences between the particles normally encountered in mining froth flotation processes, and the conditions typically encountered in froth flotation in the manufacture of recycled paper.
In order to adapt the hydrocyclone such as shown in said patents U.S. Pat. No. 4,279,743 and 4,838,434 to deinking, the consistency range of the suspension is typically limited to about 1-4%. This is a higher consistency range than in conventional deinking processes, where the suspension typically does not have a consistency of over 1.5%. This increased consistency range alone can result in increased energy efficiency and throughput is enhanced by the increased gravitional forces that exist in the hydrocyclone units utilized according to the invention. According to the invention, it is also highly desirable to deaerate the pulp discharged from the hydrocyclones, and it is necessary to adjust the amount of overflow and underflow withdrawn from the flotation units. According to the invention it is desirable to withdraw only the foam, but typically about 5-15% of the total flow of suspension is withdrawn in the rejects discharged from the hydrocyclone, while about 95-85% is withdrawn in the accepts stream.
According to one aspect of the present invention, a method of deinking recycled paper cellulosic pulp is provided, comprising the steps of sequentially and continuously: (a) Reducing paper having print thereon to a finely comminuted liquids-solids suspension flow having a low solids consistency. (b) Introducing the suspension into a generally vertical axis vortex. (c) Sparging air into the suspension in the vortex, to cause ink particles to attach to the air bubbles, and rise, a foam being formed, while cellulosic pulp moves downwardly in the vortex. And, (d) removing the foam and pulp from the vortex in a foam stream and a pulp stream distinct from the foam stream.
Step (a) of the above method is preferably practiced to provide a suspension having a solids consistency of about 1-4%, e.g. over 1.5%. Also, the following further steps are typically practiced: (e) deaerating pulp removed in the pulp stream in step (d); (f) separating the air component of the foam removed in step (d) from the solids and liquid component in the foam (by feeding the foam to a cyclone or other foam separating device); and the step (g), between steps (a) and (b), of breaking the suspension flow into a plurality of flow paths, and practicing steps (b)-(d) in each flow path, and recombining the pulp and flow streams from step (d) from all the flow paths. Steps (b)-(d) are also preferably practiced a plurality of times in in-series stages within each flow path. As earlier indicated, step (d) is preferably practiced to remove about 5-15% of the total flow of suspension in step (a) with the foam, and about 95-85% of the total flow of suspension in step (a) with the pulp.
The invention is utilized in the general process of manufacturing recycled paper, which comprises the steps of continuously: Reducing cellulosic paper having print thereon to a finely comminuted liquids-solids suspension flow having a low solids consistency. Screening and cleaning the suspension to remove reject particles. Subjecting the suspension to froth flotation to effect deinking of the cellulosic content of the suspension, to produce a deinked pulp. And, washing and thickening the cleaned pulp. Paper is made from the thickened pulp. The froth flotation step is practiced by: Introducing the suspension into a generally vertical axis vortex. Sparging air into the suspension in the vortex, to cause ink particles to rise in a foam, while cellulosic pulp moves downwardly in the vortex. And, removing the foam and pulp from the vortex in a foam stream and a pulp stream distinct from the foam stream.
The invention also contemplates apparatus for deinking paper pulp. The apparatus comprises: Means for comminuting paper having print thereon to provide a low consistency suspension, and having an outlet; and a plurality of vertical axis hydrocyclones, each having a substantially hollow body with a top and bottom, a suspension inlet adjacent the top, an interior porous wall, means for sparging air into the body by passing it through the interior wall, an overflow outlet adjacent the top, and an underflow outlet adjacent the bottom. The hydrocyclones are disposed so that a plurality of them are connected in series, with the underflow outlet from one connected to the suspension inlet of the next; and a plurality of series are provided, connected in parallel with the comminuting means outlet and a common underflow conduit. Deaerating means preferably are disposed in the common underflow conduit, for deaerating the pulp.
It is the primary object of the present invention to provide increased efficiency and throughput during the deinking of pulp by the froth flotation process. This and other objects of the invention will become clear from an inspection of the detailed description of the invention, and from the appended claims.